Jahn-Teller Effect in the EPR Spectrum of Cu++: MgO at 1.2°K

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Abstract
The EPR spectrum of Cu++: MgO at 1.2°K is described and shown to be characteristic of a Kramers quartet of states which have anisotropic transitions characteristic of a one‐ion‐per‐unit‐cell system. This is an octahedral symmetry property which may be realized either by the vanishing of the Jahn‐Teller effect at 1.2°K or by the presence of tunneling between equivalent tetragonally distorted states. The tunneling process generates a Kramers doublet and quartet separated by a small tunneling (inversion) splitting, as has been shown by Bersuker and O'Brien. A theoretical g‐value formula for the tunneling quartet is derived for the magnetic field in an arbitrary direction. The experimental g values are shown to be in good agreement with this formula, but they are not in agreement with a similar formula for the static octahedral symmetry quartet. The hyperfine structure is also shown to be better explained as a property of the tunneling quartet. The tunneling doublet has not been detected at 1.2°K and is presumed to be thermally depopulated. It is concluded that the paramagnetic resonance properties of Cu++:MgO at liquid‐helium temperatures measured at 9 × 109 cps are determined by a dynamic tunneling effect. We have found no evidence for a “frozen‐out” tetragonal distortion.

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